1. Field of the Invention
This invention relates to apparatus and a method for welding and, in particular, to hot wire gas tungsten arc welding with specific attention paid to the manner and arrangement for heating the filler wire.
2. Background Information
In hot wire-gas tungsten arc welding (HW-GTAW) the filler wire is resistance heated before it enters into the weld pool to a cherry red condition, approximately 1400.degree. F. to 1600.degree. F., or just below the melting point. An electrical current is passed through the length of the filler wire between a contact tube through which the wire is fed and the work. This length of wire acts as a resistor. Resistance heating, also referred to as I.sup.2 R heating, of the filler wire allows for higher deposition rates when compared with a normal cold wire feed GTAW machine welding process.
In order to raise the temperature of the wire close to the melting point with commercially available equipment, a high current must be applied to the wire. This causes two well documented problems, both associated with the phenomena commonly referred to as "arc blow." For discussion purposes, these two types of arc blow will be referred to as "type 1" and "type 2." Type 1 arc blow is a result of the continual deflection imposed upon the welding arc by the magnetic fields generated by the high currents needed to pre-heat the weld filler wire or Hot Wire (HW). The HW current interferes with the welding arc by deflecting it from its preferred cathode attachment point directly below the cathode, resulting in lack of fusion and other associated weld defects, i.e., hollow bead, cold lapping, lack of penetration, and so forth. Type 2 arc blow is the random arc deflection imposed upon the welding arc from the occasional breaking of the wire to puddle contact. Partly the result of the use of high HW current, the wire melts back and detaches itself from the weld pool. This in turn creates an even greater electrical interference on the welding arc as the partially molten, balled-up wire end, which is the electrical anode spot, searches for the preferential cathode, or arc attachment spot. This again results in weld quality problems, as listed above. It can cause also lead to false voltage readings when using a system with automatic voltage control (AVC) causing the torch to rise out of the weld groove and making it necessary to stop the welding process.
The best way to overcome arc blow incurred from the addition of HW to the GTAW process has been to limit the process to welding in the flat and roll positions only. To further help overcome the problems associated with the two types of arc blow, the weld wire is fed from the rear of the weld puddle. The rear wire feed method has been used because it presents a larger puddle surface to maintain the wire in contact with, and hopefully eliminate the melt back problem, or type 2 arc blow. This rear wire feed method makes it difficult for the operator to use in the field where it is necessary to view the weld puddle and feed wire to the front, welding in the forward direction. These practices, welding in the flat and roll positions, and feeding the wire from the rear, have limited the process application capability and have generally restricted the use of hot wire welding to a more controllable shop environment.
There is a need therefor for improved apparatus and method for hot wire-gas tungsten arc welding which makes it more versatile and capable of being used in all welding positions encountered in field use.
More particularly, there is a need for such an improved apparatus and method for HW-GTAW which eliminates, or at least greatly reduces, the arc blow phenomena.
There is a need for such an improved apparatus and method for HW-GTAW which still heats the HW to near the melting point to provide the capability for high deposition rates.